Replace const usage of dm_config_find_node with more appropriate value-lookup

Replace const usage of dm_config_find_node with more appropriate value-lookup
functionality. A number of bugs (copied and pasted all over the code) should
disappear:

- most string lookup based on dm_config_find_node would segfault when
  encountering a non-zero integer (the intention there was to print an
  error message instead)
- check for required sections in metadata would have been satisfied by
  values as well (i.e. not sections)
- encountering a section in place of expected flag value would have
  segfaulted (due to assumed but unchecked cn->v != NULL)